P3B-6 Implications of Mode Conversion on Transcranial Ultrasound Imaging

Transcranial ultrasound sector images are affected by a "streak artifact": two dark streaks stem radially from the apex to the maximal depth of the scan at particular angles, limiting the effective field of view and diagnostic confidence. The aim of this paper is to investigate experimentally the angle dependency of ultrasound transmission through the skull in order to understand and try to reduce this artifact. It is shown that the streaks are a consequence of the angle-dependent amplitude transmission coefficient at fluid-solid interfaces such as the interfaces between the skull bone and the surrounding fluid-like soft tissues. At a critical incidence angle of about 27deg, no ultrasound wave can cross the soft tissue-skull interfaces, which explains the presence of the streaks. At higher incidence angles, mode conversion from longitudinal to transverse waves occurs in the skull, and then back to longitudinal into the brain. The influence of the incidence angle on the distortion of the transmitted beam is also qualitatively assessed. Implications for transcranial imaging are discussed.